---
title: Setting up a Development Environment
sidebar_position: 3
version: 1
---
# Setting up a Development Environment

The documentation in this section is a bit of a patchwork of knowledge representing the
multitude of ways that exist to run Superset (`docker compose`, just "docker", on "metal", using
a Makefile).

:::note
Now we have evolved to recommend and support `docker compose` more actively as the main way
to run Superset for development and preserve your sanity. **Most people should stick to
the first few sections - ("Fork & Clone", "docker compose" and "Installing Dev Tools")**
:::

## Fork and Clone

First, [fork the repository on GitHub](https://help.github.com/articles/about-forks/),
then clone it.

Second, you can clone the main repository directly, but you won't be able to send pull requests.

```bash
git clone git@github.com:your-username/superset.git
cd superset
```

## docker compose (recommended!)

Setting things up to squeeze an "hello world" into any part of Superset should be as simple as

```bash
docker compose up
```

Note that:

- this will pull/build docker images and run a cluster of services, including:
  - A Superset **Flask web server**, mounting the local python repo/code
  - A Superset **Celery worker**, also mounting the local python repo/code
  - A Superset **Node service**, mounting, compiling and bundling the JS/TS assets
  - A Superset **Node websocket service** to power the async backend
  - **Postgres** as the metadata database and to store example datasets, charts and dashboards which
    should be populated upon startup
  - **Redis** as the message queue for our async backend and caching backend
- It'll load up examples into the database upon first startup
- all other details and pointers available in
  [docker-compose.yml](https://github.com/apache/superset/blob/master/docker-compose.yml)
- The local repository is mounted within the services, meaning updating
  the code on the host will be reflected in the docker images
- Superset is served at localhost:8088/
- You can login with admin/admin

:::caution
Since `docker compose` is primarily designed to run a set of containers on **a single host**
and can't credibly support **high availability** as a result, we do not support nor recommend
using our `docker compose` constructs to support production-type use-cases. For single host
environments, we recommend using [minikube](https://minikube.sigs.k8s.io/docs/start/) along
our [installing on k8s](https://superset.apache.org/docs/installation/running-on-kubernetes)
documentation.
configured to be secure.
:::

## Installing Development Tools

:::note
While `docker compose` simplifies a lot of the setup, there are still
many things you'll want to set up locally to power your IDE, and things like
**commit hooks**, **linters**, and **test-runners**. Note that you can do these
things inside docker images with commands like `docker compose exec superset_app bash` for
instance, but many people like to run that tooling from their host.
:::

### Python environment

Assuming you already have a way to setup your python environments
like `pyenv`, `virtualenv` or something else, all you should have to
do is to install our dev, pinned python requirements bundle

```bash
pip install -r requirements/development.txt
```

### Git Hooks

Superset uses Git pre-commit hooks courtesy of [pre-commit](https://pre-commit.com/).
To install run the following:

```bash
pre-commit install
```

This will install the hooks in your local repository. From now on, a series of checks will
automatically run whenever you make a Git commit.

#### Running Pre-commit Manually

You can also run the pre-commit checks manually in various ways:

- **Run pre-commit on all files (same as CI):**

  To run the pre-commit checks across all files in your repository, use the following command:

  ```bash
  pre-commit run --all-files
  ```

  This is the same set of checks that will run during CI, ensuring your changes meet the project's standards.

- **Run pre-commit on a specific file:**

  If you want to check or fix a specific file, you can do so by specifying the file path:

  ```bash
  pre-commit run --files path/to/your/file.py
  ```

  This will only run the checks on the file(s) you specify.

- **Run a specific pre-commit check:**

  To run a specific check (hook) across all files or a particular file, use the following command:

  ```bash
  pre-commit run <hook_id> --all-files
  ```

  Or for a specific file:

  ```bash
  pre-commit run <hook_id> --files path/to/your/file.py
  ```

  Replace `<hook_id>` with the ID of the specific hook you want to run. You can find the list
  of available hooks in the `.pre-commit-config.yaml` file.


## Alternatives to `docker compose`

:::caution
This part of the documentation is a patchwork of information related to setting up
development environments without `docker compose` and are documented/supported to varying
degrees. It's been difficult to maintain this wide array of methods and insure they're
functioning across environments.
:::

### Flask server

#### OS Dependencies

Make sure your machine meets the [OS dependencies](https://superset.apache.org/docs/installation/pypi#os-dependencies) before following these steps.
You also need to install MySQL.

Ensure that you are using Python version 3.9, 3.10 or 3.11, then proceed with:

```bash
# Create a virtual environment and activate it (recommended)
python3 -m venv venv # setup a python3 virtualenv
source venv/bin/activate

# Install external dependencies
pip install -r requirements/development.txt

# Install Superset in editable (development) mode
pip install -e .

# Initialize the database
superset db upgrade

# Create an admin user in your metadata database (use `admin` as username to be able to load the examples)
superset fab create-admin

# Create default roles and permissions
superset init

# Load some data to play with.
# Note: you MUST have previously created an admin user with the username `admin` for this command to work.
superset load-examples

# Start the Flask dev web server from inside your virtualenv.
# Note that your page may not have CSS at this point.
# See instructions below how to build the front-end assets.
superset run -p 8088 --with-threads --reload --debugger --debug
```

Or you can install via our Makefile

```bash
# Create a virtual environment and activate it (recommended)
$ python3 -m venv venv # setup a python3 virtualenv
$ source venv/bin/activate

# install pip packages + pre-commit
$ make install

# Install superset pip packages and setup env only
$ make superset

# Setup pre-commit only
$ make pre-commit
```

**Note: the FLASK_APP env var should not need to be set, as it's currently controlled
via `.flaskenv`, however if needed, it should be set to `superset.app:create_app()`**

If you have made changes to the FAB-managed templates, which are not built the same way as the newer, React-powered front-end assets, you need to start the app without the `--with-threads` argument like so:
`superset run -p 8088 --reload --debugger --debug`

#### Dependencies

If you add a new requirement or update an existing requirement (per the `install_requires` section in `setup.py`) you must recompile (freeze) the Python dependencies to ensure that for CI, testing, etc. the build is deterministic. This can be achieved via,

```bash
$ python3 -m venv venv
$ source venv/bin/activate
$ python3 -m pip install -r requirements/development.txt
$ pip-compile-multi --no-upgrade
```

When upgrading the version number of a single package, you should run `pip-compile-multi` with the `-P` flag:

```bash
$ pip-compile-multi -P my-package
```

To bring all dependencies up to date as per the restrictions defined in `setup.py` and `requirements/*.in`, run pip-compile-multi` without any flags:

```bash
$ pip-compile-multi
```

This should be done periodically, but it is recommended to do thorough manual testing of the application to ensure no breaking changes have been introduced that aren't caught by the unit and integration tests.

#### Logging to the browser console

This feature is only available on Python 3. When debugging your application, you can have the server logs sent directly to the browser console using the [ConsoleLog](https://github.com/betodealmeida/consolelog) package. You need to mutate the app, by adding the following to your `config.py` or `superset_config.py`:

```python
from console_log import ConsoleLog

def FLASK_APP_MUTATOR(app):
    app.wsgi_app = ConsoleLog(app.wsgi_app, app.logger)
```

Then make sure you run your WSGI server using the right worker type:

```bash
gunicorn "superset.app:create_app()" -k "geventwebsocket.gunicorn.workers.GeventWebSocketWorker" -b 127.0.0.1:8088 --reload
```

You can log anything to the browser console, including objects:

```python
from superset import app
app.logger.error('An exception occurred!')
app.logger.info(form_data)
```

### Frontend

Frontend assets (TypeScript, JavaScript, CSS, and images) must be compiled in order to properly display the web UI. The `superset-frontend` directory contains all NPM-managed frontend assets. Note that for some legacy pages there are additional frontend assets bundled with Flask-Appbuilder (e.g. jQuery and bootstrap). These are not managed by NPM and may be phased out in the future.

#### Prerequisite

##### nvm and node

First, be sure you are using the following versions of Node.js and npm:

- `Node.js`: Version 20
- `npm`: Version 10

We recommend using [nvm](https://github.com/nvm-sh/nvm) to manage your node environment:

```bash
curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.37.0/install.sh | bash

in case it shows '-bash: nvm: command not found'
export NVM_DIR="$HOME/.nvm"
[ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh"  # This loads nvm
[ -s "$NVM_DIR/bash_completion" ] && \. "$NVM_DIR/bash_completion"  # This loads nvm bash_completion

cd superset-frontend
nvm install --lts
nvm use --lts
```

Or if you use the default macOS starting with Catalina shell `zsh`, try:

```zsh
sh -c "$(curl -fsSL https://raw.githubusercontent.com/nvm-sh/nvm/v0.37.0/install.sh)"
```

For those interested, you may also try out [avn](https://github.com/nvm-sh/nvm#deeper-shell-integration) to automatically switch to the node version that is required to run Superset frontend.

#### Install dependencies

Install third-party dependencies listed in `package.json` via:

```bash
# From the root of the repository
cd superset-frontend

# Install dependencies from `package-lock.json`
npm ci
```

Note that Superset uses [Scarf](https://docs.scarf.sh) to capture telemetry/analytics about versions being installed, including the `scarf-js` npm package and an analytics pixel. As noted elsewhere in this documentation, Scarf gathers aggregated stats for the sake of security/release strategy, and does not capture/retain PII. [You can read here](https://docs.scarf.sh/package-analytics/) about the `scarf-js` package, and various means to opt out of it, but you can opt out of the npm package _and_ the pixel by setting the `SCARF_ANALYTICS` environment variable to `false` or opt out of the pixel by adding this setting in `superset-frontent/package.json`:

```json
// your-package/package.json
{
  // ...
  "scarfSettings": {
    "enabled": false
  }
  // ...
}
```

#### Build assets

There are three types of assets you can build:

1. `npm run build`: the production assets, CSS/JSS minified and optimized
2. `npm run dev-server`: local development assets, with sourcemaps and hot refresh support
3. `npm run build-instrumented`: instrumented application code for collecting code coverage from Cypress tests

If while using the above commands you encounter an error related to the limit of file watchers:

```bash
Error: ENOSPC: System limit for number of file watchers reached
```

The error is thrown because the number of files monitored by the system has reached the limit.
You can address this this error by increasing the number of inotify watchers.

The current value of max watches can be checked with:

```bash
cat /proc/sys/fs/inotify/max_user_watches
```

Edit the file `/etc/sysctl.conf` to increase this value.
The value needs to be decided based on the system memory [(see this StackOverflow answer for more context)](https://stackoverflow.com/questions/535768/what-is-a-reasonable-amount-of-inotify-watches-with-linux).

Open the file in editor and add a line at the bottom specifying the max watches values.

```bash
fs.inotify.max_user_watches=524288
```

Save the file and exit editor.
To confirm that the change succeeded, run the following command to load the updated value of max_user_watches from `sysctl.conf`:

```bash
sudo sysctl -p
```

#### Webpack dev server

The dev server by default starts at `http://localhost:9000` and proxies the backend requests to `http://localhost:8088`.

So a typical development workflow is the following:

1. [run Superset locally](#flask-server) using Flask, on port `8088` — but don't access it directly,<br/>

   ```bash
   # Install Superset and dependencies, plus load your virtual environment first, as detailed above.
   superset run -p 8088 --with-threads --reload --debugger --debug
   ```

2. in parallel, run the Webpack dev server locally on port `9000`,<br/>

   ```bash
   npm run dev-server
   ```

3. access `http://localhost:9000` (the Webpack server, _not_ Flask) in your web browser. This will use the hot-reloading front-end assets from the Webpack development server while redirecting back-end queries to Flask/Superset: your changes on Superset codebase — either front or back-end — will then be reflected live in the browser.

It's possible to change the Webpack server settings:

```bash
# Start the dev server at http://localhost:9000
npm run dev-server

# Run the dev server on a non-default port
npm run dev-server -- --port=9001

# Proxy backend requests to a Flask server running on a non-default port
npm run dev-server -- --env=--supersetPort=8081

# Proxy to a remote backend but serve local assets
npm run dev-server -- --env=--superset=https://superset-dev.example.com
```

The `--superset=` option is useful in case you want to debug a production issue or have to setup Superset behind a firewall. It allows you to run Flask server in another environment while keep assets building locally for the best developer experience.

#### Other npm commands

Alternatively, there are other NPM commands you may find useful:

1. `npm run build-dev`: build assets in development mode.
2. `npm run dev`: built dev assets in watch mode, will automatically rebuild when a file changes

#### Docker (docker compose)

See docs [here](https://superset.apache.org/docs/installation/docker-compose)

#### Updating NPM packages

Use npm in the prescribed way, making sure that
`superset-frontend/package-lock.json` is updated according to `npm`-prescribed
best practices.

#### Feature flags

Superset supports a server-wide feature flag system, which eases the incremental development of features. To add a new feature flag, simply modify `superset_config.py` with something like the following:

```python
FEATURE_FLAGS = {
    'SCOPED_FILTER': True,
}
```

If you want to use the same flag in the client code, also add it to the FeatureFlag TypeScript enum in [@superset-ui/core](https://github.com/apache/superset/blob/master/superset-frontend/packages/superset-ui-core/src/utils/featureFlags.ts). For example,

```typescript
export enum FeatureFlag {
  SCOPED_FILTER = "SCOPED_FILTER",
}
```

`superset/config.py` contains `DEFAULT_FEATURE_FLAGS` which will be overwritten by
those specified under FEATURE_FLAGS in `superset_config.py`. For example, `DEFAULT_FEATURE_FLAGS = { 'FOO': True, 'BAR': False }` in `superset/config.py` and `FEATURE_FLAGS = { 'BAR': True, 'BAZ': True }` in `superset_config.py` will result
in combined feature flags of `{ 'FOO': True, 'BAR': True, 'BAZ': True }`.

The current status of the usability of each flag (stable vs testing, etc) can be found in `RESOURCES/FEATURE_FLAGS.md`.

## Git Hooks

Superset uses Git pre-commit hooks courtesy of [pre-commit](https://pre-commit.com/). To install run the following:

```bash
pip3 install -r requirements/development.txt
pre-commit install
```

A series of checks will now run when you make a git commit.

Alternatively it is possible to run pre-commit via tox:

```bash
tox -e pre-commit
```

Or by running pre-commit manually:

```bash
pre-commit run --all-files
```

## Linting

### Python

We use [Pylint](https://pylint.org/) for linting which can be invoked via:

```bash
# for python
tox -e pylint
```

In terms of best practices please avoid blanket disabling of Pylint messages globally (via `.pylintrc`) or top-level within the file header, albeit there being a few exceptions. Disabling should occur inline as it prevents masking issues and provides context as to why said message is disabled.

Additionally, the Python code is auto-formatted using [Black](https://github.com/python/black) which
is configured as a pre-commit hook. There are also numerous [editor integrations](https://black.readthedocs.io/en/stable/integrations/editors.html)

### TypeScript

```bash
cd superset-frontend
npm ci
# run eslint checks
npm run eslint -- .
# run tsc (typescript) checks
npm run type
```

If using the eslint extension with vscode, put the following in your workspace `settings.json` file:

```json
"eslint.workingDirectories": [
  "superset-frontend"
]
```

## Testing

### Python Testing

All python tests are carried out in [tox](https://tox.readthedocs.io/en/latest/index.html)
a standardized testing framework.
All python tests can be run with any of the tox [environments](https://tox.readthedocs.io/en/latest/example/basic.html#a-simple-tox-ini-default-environments), via,

```bash
tox -e <environment>
```

For example,

```bash
tox -e py38
```

Alternatively, you can run all tests in a single file via,

```bash
tox -e <environment> -- tests/test_file.py
```

or for a specific test via,

```bash
tox -e <environment> -- tests/test_file.py::TestClassName::test_method_name
```

Note that the test environment uses a temporary directory for defining the
SQLite databases which will be cleared each time before the group of test
commands are invoked.

There is also a utility script included in the Superset codebase to run python integration tests. The [readme can be
found here](https://github.com/apache/superset/tree/master/scripts/tests)

To run all integration tests for example, run this script from the root directory:

```bash
scripts/tests/run.sh
```

You can run unit tests found in './tests/unit_tests' for example with pytest. It is a simple way to run an isolated test that doesn't need any database setup

```bash
pytest ./link_to_test.py
```

### Frontend Testing

We use [Jest](https://jestjs.io/) and [Enzyme](https://airbnb.io/enzyme/) to test TypeScript/JavaScript. Tests can be run with:

```bash
cd superset-frontend
npm run test
```

To run a single test file:

```bash
npm run test -- path/to/file.js
```

### Integration Testing

We use [Cypress](https://www.cypress.io/) for integration tests. Tests can be run by `tox -e cypress`. To open Cypress and explore tests first setup and run test server:

```bash
export SUPERSET_CONFIG=tests.integration_tests.superset_test_config
export SUPERSET_TESTENV=true
export CYPRESS_BASE_URL="http://localhost:8081"
superset db upgrade
superset load_test_users
superset load-examples --load-test-data
superset init
superset run --port 8081
```

Run Cypress tests:

```bash
cd superset-frontend
npm run build-instrumented

cd cypress-base
npm install

# run tests via headless Chrome browser (requires Chrome 64+)
npm run cypress-run-chrome

# run tests from a specific file
npm run cypress-run-chrome -- --spec cypress/e2e/explore/link.test.ts

# run specific file with video capture
npm run cypress-run-chrome -- --spec cypress/e2e/dashboard/index.test.js --config video=true

# to open the cypress ui
npm run cypress-debug

# to point cypress to a url other than the default (http://localhost:8088) set the environment variable before running the script
# e.g., CYPRESS_BASE_URL="http://localhost:9000"
CYPRESS_BASE_URL=<your url> npm run cypress open
```

See [`superset-frontend/cypress_build.sh`](https://github.com/apache/superset/blob/master/superset-frontend/cypress_build.sh).

As an alternative you can use docker compose environment for testing:

Make sure you have added below line to your /etc/hosts file:
`127.0.0.1 db`

If you already have launched Docker environment please use the following command to assure a fresh database instance:
`docker compose down -v`

Launch environment:

`CYPRESS_CONFIG=true docker compose up`

It will serve backend and frontend on port 8088.

Run Cypress tests:

```bash
cd cypress-base
npm install
npm run cypress open
```

### Debugging Server App

#### Local

For debugging locally using VSCode, you can configure a launch configuration file .vscode/launch.json such as

```json
{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Python: Flask",
      "type": "python",
      "request": "launch",
      "module": "flask",
      "env": {
        "FLASK_APP": "superset",
        "SUPERSET_ENV": "development"
      },
      "args": ["run", "-p 8088", "--with-threads", "--reload", "--debugger"],
      "jinja": true,
      "justMyCode": true
    }
  ]
}
```

#### Raw Docker (without `docker compose`)

Follow these instructions to debug the Flask app running inside a docker container. Note that
this will run a barebones Superset web server,

First add the following to the ./docker-compose.yaml file

```diff
superset:
    env_file: docker/.env
    image: *superset-image
    container_name: superset_app
    command: ["/app/docker/docker-bootstrap.sh", "app"]
    restart: unless-stopped
+   cap_add:
+     - SYS_PTRACE
    ports:
      - 8088:8088
+     - 5678:5678
    user: "root"
    depends_on: *superset-depends-on
    volumes: *superset-volumes
    environment:
      CYPRESS_CONFIG: "${CYPRESS_CONFIG}"
```

Start Superset as usual

```bash
docker compose up
```

Install the required libraries and packages to the docker container

Enter the superset_app container

```bash
docker exec -it superset_app /bin/bash
root@39ce8cf9d6ab:/app#
```

Run the following commands inside the container

```bash
apt update
apt install -y gdb
apt install -y net-tools
pip install debugpy
```

Find the PID for the Flask process. Make sure to use the first PID. The Flask app will re-spawn a sub-process every time you change any of the python code. So it's important to use the first PID.

```bash
ps -ef

UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 14:09 ?        00:00:00 bash /app/docker/docker-bootstrap.sh app
root         6     1  4 14:09 ?        00:00:04 /usr/local/bin/python /usr/bin/flask run -p 8088 --with-threads --reload --debugger --host=0.0.0.0
root        10     6  7 14:09 ?        00:00:07 /usr/local/bin/python /usr/bin/flask run -p 8088 --with-threads --reload --debugger --host=0.0.0.0
```

Inject debugpy into the running Flask process. In this case PID 6.

```bash
python3 -m debugpy --listen 0.0.0.0:5678 --pid 6
```

Verify that debugpy is listening on port 5678

```bash
netstat -tunap

Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:5678            0.0.0.0:*               LISTEN      462/python
tcp        0      0 0.0.0.0:8088            0.0.0.0:*               LISTEN      6/python
```

You are now ready to attach a debugger to the process. Using VSCode you can configure a launch configuration file .vscode/launch.json like so.

```json
{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Attach to Superset App in Docker Container",
      "type": "python",
      "request": "attach",
      "connect": {
        "host": "127.0.0.1",
        "port": 5678
      },
      "pathMappings": [
        {
          "localRoot": "${workspaceFolder}",
          "remoteRoot": "/app"
        }
      ]
    }
  ]
}
```

VSCode will not stop on breakpoints right away. We've attached to PID 6 however it does not yet know of any sub-processes. In order to "wake up" the debugger you need to modify a python file. This will trigger Flask to reload the code and create a new sub-process. This new sub-process will be detected by VSCode and breakpoints will be activated.

### Debugging Server App in Kubernetes Environment

To debug Flask running in POD inside a kubernetes cluster, you'll need to make sure the pod runs as root and is granted the SYS_TRACE capability.These settings should not be used in production environments.

```yaml
  securityContext:
    capabilities:
      add: ["SYS_PTRACE"]
```

See (set capabilities for a container)[https://kubernetes.io/docs/tasks/configure-pod-container/security-context/#set-capabilities-for-a-container] for more details.

Once the pod is running as root and has the SYS_PTRACE capability it will be able to debug the Flask app.

You can follow the same instructions as in `docker compose`. Enter the pod and install the required library and packages; gdb, netstat and debugpy.

Often in a Kubernetes environment nodes are not addressable from outside the cluster. VSCode will thus be unable to remotely connect to port 5678 on a Kubernetes node. In order to do this you need to create a tunnel that port forwards 5678 to your local machine.

```bash
kubectl port-forward  pod/superset-<some random id> 5678:5678
```

You can now launch your VSCode debugger with the same config as above. VSCode will connect to to 127.0.0.1:5678 which is forwarded by kubectl to your remote kubernetes POD.

### Storybook

Superset includes a [Storybook](https://storybook.js.org/) to preview the layout/styling of various Superset components, and variations thereof. To open and view the Storybook:

```bash
cd superset-frontend
npm run storybook
```

When contributing new React components to Superset, please try to add a Story alongside the component's `jsx/tsx` file.

## Tips

### Adding a new datasource

1. Create Models and Views for the datasource, add them under superset folder, like a new my_models.py
   with models for cluster, datasources, columns and metrics and my_views.py with clustermodelview
   and datasourcemodelview.

1. Create DB migration files for the new models

1. Specify this variable to add the datasource model and from which module it is from in config.py:

   For example:

   ```python
   ADDITIONAL_MODULE_DS_MAP = {'superset.my_models': ['MyDatasource', 'MyOtherDatasource']}
   ```

   This means it'll register MyDatasource and MyOtherDatasource in superset.my_models module in the source registry.

### Visualization Plugins

The topic of authoring new plugins, whether you'd like to contribute
it back or not has been well documented in the
[the documentation](https://superset.apache.org/docs/contributing/creating-viz-plugins), and in [this blog post](https://preset.io/blog/building-custom-viz-plugins-in-superset-v2).

To contribute a plugin to Superset, your plugin must meet the following criteria:

- The plugin should be applicable to the community at large, not a particularly specialized use case
- The plugin should be written with TypeScript
- The plugin should contain sufficient unit/e2e tests
- The plugin should use appropriate namespacing, e.g. a folder name of `plugin-chart-whatever` and a package name of `@superset-ui/plugin-chart-whatever`
- The plugin should use theme variables via Emotion, as passed in by the ThemeProvider
- The plugin should provide adequate error handling (no data returned, malformed data, invalid controls, etc.)
- The plugin should contain documentation in the form of a populated `README.md` file
- The plugin should have a meaningful and unique icon
- Above all else, the plugin should come with a _commitment to maintenance_ from the original author(s)

Submissions will be considered for submission (or removal) on a case-by-case basis.

### Adding a DB migration

1. Alter the model you want to change. This example will add a `Column` Annotations model.

   [Example commit](https://github.com/apache/superset/commit/6c25f549384d7c2fc288451222e50493a7b14104)

1. Generate the migration file

   ```bash
   superset db migrate -m 'add_metadata_column_to_annotation_model'
   ```

   This will generate a file in `migrations/version/{SHA}_this_will_be_in_the_migration_filename.py`.

   [Example commit](https://github.com/apache/superset/commit/d3e83b0fd572c9d6c1297543d415a332858e262)

1. Upgrade the DB

   ```bash
   superset db upgrade
   ```

   The output should look like this:

   ```log
   INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
   INFO  [alembic.runtime.migration] Will assume transactional DDL.
   INFO  [alembic.runtime.migration] Running upgrade 1a1d627ebd8e -> 40a0a483dd12, add_metadata_column_to_annotation_model.py
   ```

1. Add column to view

   Since there is a new column, we need to add it to the AppBuilder Model view.

   [Example commit](https://github.com/apache/superset/pull/5745/commits/6220966e2a0a0cf3e6d87925491f8920fe8a3458)

1. Test the migration's `down` method

   ```bash
   superset db downgrade
   ```

   The output should look like this:

   ```log
   INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
   INFO  [alembic.runtime.migration] Will assume transactional DDL.
   INFO  [alembic.runtime.migration] Running downgrade 40a0a483dd12 -> 1a1d627ebd8e, add_metadata_column_to_annotation_model.py
   ```

### Merging DB migrations

When two DB migrations collide, you'll get an error message like this one:

```text
alembic.util.exc.CommandError: Multiple head revisions are present for
given argument 'head'; please specify a specific target
revision, '<branchname>@head' to narrow to a specific head,
or 'heads' for all heads`
```

To fix it:

1. Get the migration heads

   ```bash
   superset db heads
   ```

   This should list two or more migration hashes. E.g.

   ```bash
   1412ec1e5a7b (head)
   67da9ef1ef9c (head)
   ```

2. Pick one of them as the parent revision, open the script for the other revision
   and update `Revises` and `down_revision` to the new parent revision. E.g.:

   ```diff
   --- a/67da9ef1ef9c_add_hide_left_bar_to_tabstate.py
   +++ b/67da9ef1ef9c_add_hide_left_bar_to_tabstate.py
   @@ -17,14 +17,14 @@
   """add hide_left_bar to tabstate

   Revision ID: 67da9ef1ef9c
   -Revises: c501b7c653a3
   +Revises: 1412ec1e5a7b
   Create Date: 2021-02-22 11:22:10.156942

   """

   # revision identifiers, used by Alembic.
   revision = "67da9ef1ef9c"
   -down_revision = "c501b7c653a3"
   +down_revision = "1412ec1e5a7b"

   import sqlalchemy as sa
   from alembic import op
   ```

   Alternatively you may also run `superset db merge` to create a migration script
   just for merging the heads.

   ```bash
   superset db merge {HASH1} {HASH2}
   ```

3. Upgrade the DB to the new checkpoint

   ```bash
   superset db upgrade
   ```
